Antifriction screw drive systems are well-known, in which rolling elements are interposed between a screw and nut pair to enable the system to operate at low friction and high efficiency. The most common antifriction screw drive system is the conventional ballscrew and companion ball nut, an example of which is shown in the sectional view of prior art FIG. 1, herein. In FIG. 1, the nut 10 is affixed, for example, to a moving member, while the ballscrew 11 is supported by a stationary member for rotation about the screw's longitudinal axis 12. The ballscrew 11 has a semi-circular groove 13 machined into its outer diameter 14, the groove 13 being of constant lead (i.e., constant axial advancement per revolution), and the nut 10 has a mating semi-circular groove 15 machined within its bore 16; balls 17 disposed within the grooves 13,15 of the nut 10 and screw 11 provide the connection. The pitch diameter 18, an imaginary cylinder along which the centers of the balls 17 are disposed, is between the screw's outer diameter 14 and the nut bore 16. This conventional structure for a ballscrew facilitates ease of manufacture and assembly. The cross-sectional shapes of the grooves may vary from that which is completely semi-circular yet of slightly larger radius than the balls, which will result in a point contact of the ball with the groove, to a gothic arch shape, i.e., where two separate radii are used to form the groove and, with preload (e.g., that provided by use of oversized balls), the ball will contact two points, one at each side of the groove in a manner well-known in the ballscrew art.
Many, perhaps most, conventional ballscrew assemblies employ an endless ball circuit, in which balls are guided out of the ball groove at one end of the nut, and routed back to the other end of the nut for re-entry into the ball groove. Often balls are routed through an external return tube, and sometimes they are routed through a return hole drilled through the nut body. In still other instances, balls have been routed through a-return passageway within the screw.
German patent application 2352454, published May 22, 1974 illustrates a somewhat unconventional ballscrew drive entitled Ball and Screw Gear. This prior art device, depicted in FIG. 2, herein, proposes a ballscrew and nut combination 19 where the resulting nut 20 has a reduced outer envelope, when compared with prior ball nuts which use external ball returns or ball returns passing through an axial hole in the ball nut. To accomplish the desired result, the screw 21, rotatable about its longitudinal axis 22, is provided with a thin helical thread 23 which intermeshes with, but does not touch, the thin helical thread 24 of the nonrotating nut 20; i.e., the threads 23,24 are very thin when compared to the pitch (pitch being the axial distance between corresponding points on the thread). The threads 23,24 are sized such that when in mesh, two chambers 25,26 are formed between the screw thread flanks 27,28 and nut thread flanks 29,30 so that balls 31 will migrate under thrust load through one of the chambers 25 as the drive is in motion; the second, slightly larger chamber 26 provides a return path for the balls 31 circulating back to the start of the nut 20. When reversing motion, there is a slight relative axial shift between the screw 21 and nut 20, as the clearance is taken up, and the balls 31 in the second chamber 26 become the load bearing balls. Accordingly, the formerly load bearing ball chamber 25 now becomes the clearance chamber for the recirculating balls 31. Obviously, this device has certain inherent drawbacks; e.g., (1) a lost motion occurs when reversing direction, and (2) as those skilled in the art will appreciate, the load bearing balls of the relatively rotating screw migrate at a speed less than the pitch line speed of the screw, but in the same direction, whereas the returning balls in the second chamber are moving in a direction opposite to that of the screw, and would, presumably, experience skidding and consequent wear.
One problem with many conventional ballscrew assemblies is the fact that when thrusting in one direction, the ball and respective groove of the screw and nut have single point contact which can result in great wear of the assembly and consequent degradation of the elements.
The invention contained herein obviates many of the problems associated with prior art screw and nut systems. In a preferred embodiment, helical flights are provided on a screw, and within a nut. A plurality of balls are seated within a ball track, formed by suitable ball grooves manufactured within oppositely disposed faces of the respective flights. The result is a screw system with, generally, more thrust load carrying capability and with more contact points to reduce wear.